Search for music notation software on the Internet. Select from the plethora of choices you discover. There is even free software for writing sheet music. (See References)
Download the software of your choosing. Wait until the entire file is fully downloaded before attempting to open it. Open the file and spend time carefully reading the instructions provided. Note that each piece of music notation software has its own instructions; they vary from one to the next.
Having thoroughly familiarized yourself with the instructions for the software you are using, start writing your musical composition. You can share the final product with students, publishers and producers if you like.